A typical implantable pacemaker/cardioverter/defibrillator (PCD) device has the capability of providing a variety of anti-tachycardia pacing (ATP) regimens. Normally, these regimens are applied according to a pre-programmed sequence, and each regimen includes a predetermined number of pacing pulses. After the series of pacing pulses is delivered, the device checks to determine whether the series of pulses was effective in terminating the detected tachycardia. Typically, termination is confirmed by a return to either a sinus rhythm or demand-paced rhythm, in which successive spontaneous depolarizations are separated by at least a defined interval. If the tachycardia is not terminated, the PCD device may deliver a subsequent series of pacing pulses having modified pulse parameters, e.g. reduced inter-pulse intervals and/or an altered number of pulses. When ATP attempts fail to terminate the tachycardia, high-voltage cardioversion shocks may be delivered. Since shocks can be painful to the patient and consume relatively greater battery charge than pacing pulses, it is desirable to avoid delivering unnecessary shocks.
The success of a tachycardia therapy depends in part on the accuracy of the tachycardia detection. In some cases, a tachycardia originating in the atria, i.e. a supraventricular tachycardia (SVT), is difficult to distinguish from a tachycardia originating in the ventricles, i.e. a ventricular tachycardia (VT). For example, both the atrial chambers and the ventricular chambers may exhibit a similar tachycardia cycle length when an SVT is conducted to the ventricles or a VT is conducted retrograde to the atria. Accordingly, methods are needed for accurately classifying a detected tachycardia as VT or SVT to allow the most appropriate therapy to be delivered by the PCD, with the highest likelihood of success and without unacceptably delaying attempts at terminating the tachycardia.